The present invention relates to controlling the production of a liquefied natural gas product stream obtained by removing heat from natural gas in a heat exchanger, wherein the natural gas passes through one set of tubes located in the shell side of the heat exchanger. In the heat exchanger, the natural gas is in indirect heat exchange with expanded heavy mixed refrigerant and expanded light mixed refrigerant. The heavy mixed refrigerant and the light mixed refrigerant circulate in a closed refrigeration cycle, which includes the shell side of the heat exchanger, a compressor, a cooler, a separator, two additional sets of tubes in the heat exchanger and two expansion devices debauching into the shell side, wherein the heavy mixed refrigerant and the light mixed refrigerants are produced as the liquid product and the vapour product from the separator, respectively. In the shell side of the heat exchanger, the expanded heavy mixed refrigerant and the expanded light mixed refrigerants are allowed to evaporate so as to remove heat from the natural gas passing through the one set of tubes and from the heavy and light mixed refrigerant passing through the two additional sets of tubes in the heat exchanger.
The heat exchanger can be a spoolwound heat exchanger or a plate fin heat exchanger. In the specification and in the claims the term shell side is used to refer to the cold side of the heat exchanger and the terms tube and tube bundle are used to refer to the warm side of the heat exchanger.
European patent application publication No. 893 665 discloses in FIGS. 4 and 5 a method of controlling the production of a liquefied natural gas product stream, which method comprises the steps of:
a) measuring the flow rate and the temperature of the liquefied natural gas, and measuring the flow rates of the heavy mixed refrigerant and of the light mixed refrigerant;
b) maintaining the flow rate of the liquefied natural gas product stream at an operator manipulated set point and maintaining the temperature of the liquefied natural gas product stream at an operator manipulated set point, wherein maintaining the temperature of the liquefied natural gas product stream at its operator manipulated set point comprises the steps of:
b1) determining a dependent set point for the total mixed refrigerant flow rate, the dependent set point being the sum of (i) an incremental change of the flow rate of the total mixed refrigerant to offset a difference between the temperature of the liquefied natural gas product stream and the operator manipulated set point for the temperature and (ii) the product of the operator manipulated set point for the flow rate of the liquefied natural gas product stream and the ratio of the flow rate of the total mixed refrigerant to the flow rate of the liquefied natural gas product stream (which ratio has a given value);
b2) determining a dependent set point for the light mixed refrigerant flow rate that is equal to the dependent set point for the flow rate of the total mixed refrigerant divided by the sum of 1 (=unity) and the operator manipulated set point for the ratio of the flow rate of the light mixed refrigerant to the flow rate of the heavy mixed refrigerant, and determining a dependent set point for the heavy mixed refrigerant that is the difference between the dependent set point for the flow rate of the total mixed refrigerant and the dependent set point for the light mixed refrigerant flow rate; and
b3) maintaining the light mixed refrigerant flow rate and the heavy mixed refrigerant flow rate at their dependent set points.
In this method the flow rate of the liquefied natural gas product stream and its temperature are independently controlled, and the flow rate of the total mixed refrigerant is a dependent variable. As a consequence, the maximum available power from the turbines that drive the compressors cannot be fully utilized.
It is therefore an object of the present invention to provide a method of controlling the production of a liquefied natural gas product stream wherein the temperature of the liquefied natural gas product stream and the flow rate of the mixed refrigerant are controlled, such that the flow rate of the liquefied natural gas product stream is a dependent variable.